Oscillatory Transpiration of Avena Plants: Perturbation Experiments Provide Evidence for a Stable Point of Singularity

Oscillatory plant water regulation of young Avena plants was studied. The period of the oscillations was around 40 min. Pulse perturbations were given to plants showing oscillations in the transpiration rate. Perturbations consisted in temporary irradiance changes of the leaf or in water potential changes around the root. The effect of the pulse perturbations on the amplitude of the oscillations was recorded. The oscillations could be permanently halted after a perturbation of suitable magnitude given at a suitable phase of The oscillations. A subsequent perturbation could restart the oscillations again. By means of simulations it was shown that a feedback model For the transpiration oscillations could explain the experimental outcome if a non finearity of a special kind was incorporated. The circadian eclosion rhythm of Drosophila pseudoobscura and the petal rhythm of Kalanchoe blossfeldiana show many features in common with the experiments reported. Biophysically the present results indicate that the transpiration oscillations of Avena plants have a stable point of equilibrium or a stable point of singularity.     

Oscillatory Transpiration in a Cotton Plant: I. EXPERIMENTAL CHARACTERIZATION

A resistance-capacitance model of oscillatory transpirationin a cotton plant under constant environmental conditions isdiscussed. Experimental results showed that a cotton plant exposedto a sudden and large evaporative demand went through a periodof self-adjustment indicating the presence of negative feedbackmechanisms. Phase relationships between flow in stem, leaf waterpotential, water uptake by roots, and leaf turgor were characterizedthrough the day. The water-based oscillatory processes are explainedthrough a proposed model based upon a hysteresis-controlledfunction and delayed coupling in the leaf component. The periodof oscillation was found to be about 30 min. Results also indicatedthat the lag between the transpiration and water uptake ratesduring oscillations was strongly influenced by the capacitanceof the leaf and that stem capacitance was negligible.     

Oscillations in stomatal conductance and plant functioning associated with stomatal conductance: Observations and a model

Summary Measurements of transpiration, leaf water content, and flux of water in a cotton plant exhibiting sustained oscillations, in stomatal conductance are presented, and a model of the mechanism causing this behaviour is developed. The dynamic elements, of the model are capacitors—representing the change of water content with water potential in mesophyll, subsidiary and guard cells—interconnected by resistances representing flow paths in the plant. Increase of water potential in guard cells causes an increase in stomatal conductance. Increase of water potential in the subsidiary cells has the opposite effect and provides the positive feed-back which can cause stomatal conductance to oscillate. The oscillations are shown to have many of the characteristics of free-running oscillations in real plants. The behaviour of the model has been examined, using an analogue computer, with constraints and perturbations representing some of those which could be applied to real plants in physiological experiments. Aspects of behaviour which have been simulated are (a) opening and closing of stomata under the influence of changes in illumination, (b) transient responses due to step changes in potential transpiration, root permeability and potential of water surrounding the roots, (c) the influence of these factors on the occurrence and shape of spontaneous oscillations, and (d) modulation of sustained oscillations due to a circadian rhythm in the permeability of roots.     

Rapid Oscillations in Plant Water Potential Measured with a Stem Psychrometer

Oscillations in water potential were measured for the firsttime in the plant stem. The phase of the oscillations in waterpotential (measured in Brussels sprout) lagged behind that oftranspiration by less than 5 min. The cycles ranged from 22to 48 min. The results support the view that oscillations inwater potential in the plant stem and interconnecting xylemtissues would synchronize oscillations in transpiration andstomatal opening throughout the plant. Brassica oleraceae L., Brussels sprout, oscillations, stem water potential, transpiration, stem psychrometer     

Oscillatory Transpiration and Water Uptake of Avena Plants

The oscillatory transpiration of 6 days old Avena plants was investigated with respect to the water potential of the root medium. The desired water potential was obtained by means of mannitol solutions. When the water potential was lowered (“mannitol step”), the amplitude of the oscillations decreased. Below –3.0 bars no oscillations persisted. A detailed study was made of the phase changes of the oscillations caused by a short time decrease of the water potential of the root medium (“mannitol pulse”). The duration of these short term treatments was either 9.0, 3.0 or J.5 min. The experimental results are discussed on the basis of an electric analogue previously presented in the literature. Published simulations based on the model were in clear contrast to the present experimental results as well as to earlier results in the literature. However, simulations in the present paper showed that the model could explain the experimental results if suitable parameter values were chosen.     

A new method to measure plant water uptake and transpiration simultaneously

A new weighing lysimeter system is described measuring transpirationand water uptake simultaneously on one plant, growing in waterculture. The measurements may be made for short time intervals(min) making it possible to monitor quick responses to changingenvironmental conditions. Fresh weight change, a combinationof growth and water status alterations in the plant, may becalculated from transpiration and water uptake. The system consistsof two communicating vessels filled with nutrient solution;each placed on an electronic balance. One of these vessels carriesthe plant and is connected to the other by a flexible tube.Water uptake will cause an equal decrease of the solution levelin each vessel. The weight decrease of the vessel with no plantprovides a measure of water uptake, the total weight decreaseon both balances represents transpiration. Test observations showed that measurements of transpirationand water uptake in a greenhouse can be made to an accuracyof about 0.03 g min^–1 plant^–1. With fluctuatingradiation, a clearly radiation-dependent transpiration was measuredon a tomato plant in a greenhouse. These measurements showeda delay between transpiration and water uptake. Consequently,fresh weight also fluctuated with radiation. An immediate decreasein transpiration was measured upon closure of a screen in thegreenhouse, accompanied by an increase in fresh weight. Fromlate afternoon until sunrise a constant fresh weight increasewas measured; first at a relatively high rate probably due togrowth and recovering from water deficits, thereafter at a constantrate probably only due to growth. Key words: Transpiration, water uptake, water relations, tomato, lysimeter     

Regulation of root water uptake under abiotic stress conditions

A common effect of several abiotic stresses is to cause tissue dehydration. Such dehydration is caused by the imbalance between root water uptake and leaf transpiration. Under some specific stress conditions, regulation of root water uptake is more crucial to overcome stress injury than regulation of leaf transpiration. This review first describes present knowledge about how water is taken up by roots and then discusses how specific stress situations such as drought, salinity, low temperature, and flooding modify root water uptake. The rate of root water uptake of a given plant is the result of its root hydraulic characteristics, which are ultimately regulated by aquaporin activity and, to some extent, by suberin deposition. Present knowledge about the effects of different stresses on these features is also summarized. Finally, current findings regarding how molecular signals such as the plant hormones abscisic acid, ethylene, and salicylic acid, and how reactive oxygen species may modulate the final response of root water uptake under stress conditions are discussed.     

Oscillatory Transpiration and Water Uptake of Avena Plants

The transpiration rate of oat plants, 6 days old, has been investigated. Dependent on the irradiance level of the white light used in the experiments, the transpiration rate oscillated with different period times. In darkness or at low irradiances the period was about 100–110 min. At higher irradiances the period was about 40 min. At intermediate irradiances autocorrelation analysis was used to find the period content of the transpiration rate. It was concluded that two oscillatory systems were present in the plants, characterized by their different periods. When plants cultivated in a light/dark cycle were used, the transpiration oscillations were influenced by a circadian rhythm. Oscillations in darkness were then most pronounced in the mornings. Plants cultivated in continuous light did not show such a circadian rhythm, but the oscillations died out after about 20 h. Kinetin induced transpiration oscillations in darkness and made them sustain for a longer time.     

Transpiration and water uptake of Senecio medley-woodii and Aloe jucunda under changing environmental conditions: measurements with a potometric water-budget-meter

Transpiration, water uptake by the roots and CO₂ exchange oftwo leaf succulents, Senecio medleywoodii (Asteraceae) and Aloejucunda (Asphodeliaceae), were monitored simultaneously andcontinuously with a gas exchange cuvette combined with an apparatusto quantify water uptake (= waterbudget- meter). Measurements,which are primarily valid for plants with a sufficient watersupply, were made with the same plant for up to 29 consecutivedays. Ambient air temperature varied between 17 and 35 C witha constant dewpoint temperature of 13C of the ambient air anda 12 h photoperiod at 400–500mol m^–2s^–1 photonirradiance. The net water flux (J_w(net)=water uptake–transpiration)and the water balance (J_w(net) integrated for a timespan) werecalculated. Various tests were made to determine the accuracyof the measurements made with this rather complex equipment.In most cases the errors for transpiration and uptake rateswere much lower than 8% determined under the conditions of drastically(about 10 K per 30 min) increased or decreased ambient air temperatures.The experimental set-up proved to be a most valuable tool todetermine and analyse interactions between transpiration andwater uptake, changes in plant water status and the bufferingof negative J_wnet). Increasing the temperature of ambient air resulted, for bothspecies investigated, in a quick and considerably enhanced transpiration,but there was only a minor impact on water uptake. Water lossexceeding uptake was buffered by internal water reserves whichwere refilled within about 1 d after the plant was relievedof heat and drought stress caused by a period of high ambientair temperatures and high water vapour saturation deficits ofthe air. Repeated simulation of such stress periods showed thatthe absolute values of transpiration and the water uptake for24 h can vary, but the diurnal course of the values showed thesame pattern if the environmental conditions were identical.Such standardized diurnal transpiration and water uptake curvescould be very useful for the validation of mathematical modelsused to describe plant water relations. Key words: Plant water relations, water budget, drought stress, transpiration, water uptake     

Whole-tree level water balance and its implications on stomatal oscillations in orange trees [Citrus sinensis (L.) Osbeck] under natural climatic conditions

Sustained cyclic oscillations in stomatal conductance, leaf water potential, and sap flow were observed in young orange trees growing under natural conditions. The oscillations had an average period of approximately 70 min. Water uptake by the roots and loss by the leaves was characterized by large time lags which led to imbalances between water supply and demand in the leaves. The bulk of the lag in response between stomatal movements and the upstream water balance resided downstream of the branch, with branch level sap flow lagging behind the stomatal conductance by approximately 20 min while the stem sap flow had a much shorter time lag of only 5 min behind the branch sap flow. This imbalance between water uptake and loss caused transient changes in internal water deficits which were closely correlated to the dynamics of the leaf water potential. The hydraulic resistance of the whole tree fluctuated throughout the day, suggesting transient changes in the efficiency of water supply to the leaves. A simple whole-tree water balance model was applied to describe the dynamics of water transport in the young orange trees, and typical values of the hydraulic parameters of the transpiration stream were estimated. In addition to the hydro-passive stomatal movements, whole-tree water balance appears to be an important factor in the generation of stomatal oscillations.     

Studies on water transport through the sweet cherry fruit surface: IX. Comparing permeability in water uptake and transpiration

Water uptake and transpiration were studied through the surface of intact sweet cherry (Prunus avium L.) fruit, exocarp segments (ES) and cuticular membranes (CM) excised from the cheek of sweet cherry fruit and astomatous CM isolated from Schefflera arboricola (Hayata) Hayata, Citrus aurantium L., and Stephanotis floribunda Brongn. leaves or from Lycopersicon esculentum Mill. and Capsicum annuum L. var. annuum Fasciculatum Group fruit. ES and CM were mounted in diffusion cells. Water (deionized) uptake into intact sweet cherry fruit, through ES or CM interfacing water as a donor and a polyethyleneglycol (PEG 6000, osmotic pressure 2.83 MPa)-containing receiver was determined gravimetrically. Transpiration was quantified by monitoring weight loss of a PEG 6000-containing donor (2.83 MPa) against dry silica as a receiver. The permeability coefficients for osmotic water uptake and transpiration were calculated from the amount of water taken up or transpired per unit surface area and time, and the driving force for transport. Permeability during osmotic water uptake was markedly higher than during transpiration in intact sweet cherry fruit (40.2-fold), excised ES of sweet cherry fruit (12.5- to 53.7-fold) and isolated astomatous fruit and leaf CM of a range of species (on average 23.0-fold). Partitioning water transport into stomatal and cuticular components revealed that permeability of the sweet cherry fruit cuticle for water uptake was 11.9-fold higher and that of stomata 56.8-fold higher than the respective permeability during transpiration. Increasing water vapor activity in the receiver from 0 to 1 increased permeability during transpiration across isolated sweet cherry fruit CM about 2.1-fold. Permeability for vapor uptake from saturated water vapor into a PEG 6000 receiver solution was markedly lower than from liquid water, but of similar magnitude to the permeability during self-diffusion of ³H₂O in the absence of osmotica. The energy of activation for self-diffusion of water across ES or CM was higher than for osmotic water uptake and decreased with increasing stomatal density. The data indicate that viscous flow along an aqueous continuum across the sweet cherry fruit exocarp and across the astomatous CM of selected species accounted for the higher permeability during water uptake as compared to self-diffusion or transpiration.     

Oscillatory Transpiration and Water Uptake of Avena Plants

Xylem vessels in the lower part of the leaf of young Avena plants have been exposed to deformation by application of an external pressure. In this way a resistance to the water flow at the deformation site has been achieved, inducing undamped oscillations in transpiration and water uptake, even after removal of the root system.     

Alternating Perturbation Response of the Water Regulatory System in Arena Leaves

The transpiration response of excised primary Avena leaves was studied when pulse perturbations were given to the water regulatory system. Repeated light pulses given to the leaf caused regularly alternating transpiration responses, i.e. the magnitude alternated regularly between a high and a low value. This effect, denoted alternating pulse response, could be recorded under quite different light pulse conditions but was not found when the pulse interval was too long or too short (longer than about 60 min. shorter than about 15 min). Sodium chloride given to the transpiration stream induced and increased the effect. Alternating pulse response could also be recorded when mannitol pulses were given to the root system of intact plants.     

Co-ordination of vapour and liquid phase water transport properties in plants

The pathway for water movement from the soil through plants to the atmosphere can be represented by a series of liquid and vapour phase resistances. Stomatal regulation of vapour phase resistance balances transpiration with the efficiency of water supply to the leaves, avoiding leaf desiccation at one extreme, and unnecessary restriction of carbon dioxide uptake at the other. In addition to maintaining a long-term balance between vapour and liquid phase water transport resistances in plants, stomata are exquisitely sensitive to short-term, dynamic perturbations of liquid water transport. In balancing vapour and liquid phase water transport, stomata do not seem to distinguish among potential sources of variation in the apparent efficiency of delivery of water per guard cell complex. Therefore, an apparent soil-to-leaf hydraulic conductance based on relationships between liquid water fluxes and driving forces in situ seems to be the most versatile for interpretation of stomatal regulatory behaviour that achieves relative homeostasis of leaf water status in intact plants. Components of dynamic variation in apparent hydraulic conductance in intact plants include, exchange of water between the transpiration stream and internal storage compartments via capacitive discharge and recharge, cavitation and its reversal, temperature-induced changes in the viscosity of water, direct effects of xylem sap composition on xylem hydraulic properties, and endogenous and environmentally induced variation in the activity of membrane water channels in the hydraulic pathway. Stomatal responses to humidity must also be considered in interpreting co-ordination of vapour and liquid phase water transport because homeostasis of bulk leaf water status can only be achieved through regulation of the actual transpirational flux. Results of studies conducted with multiple species point to considerable convergence with regard to co-ordination of stomatal and hydraulic properties. Because stomata apparently sense and respond to integrated and dynamic soil-to-leaf water transport properties, studies involving intact plants under both natural and controlled conditions are likely to yield the most useful new insights concerning stomatal co-ordination of transpiration with soil and plant hydraulic properties.     

Water balance of plants during root application of high concentrations of growth substances

Using hydroponic cultures, the effect of high concentrations (10^?3 m) of 2-methyl-4-chlorophenoxyacetic acid (MCPA) in the root medium on the water balance of 8–9 week old plants ofPisum sativum L. and of 9–10 week-old plants ofSinapis alba L. was studied. The water balance was determined in the light and in the dark gravimetrically by measuring the intensity of water uptake and transpiration in plants cultivated by the method of root bridges according to Werner. MCPA present in the root medium in illuminated plants decreased rapidly the intensity of both the uptake and the loss components of the water balance. In permanent darkness, MCPA brought about an increase in the intensity of uptake of water and of transpiration. Simultaneous determination of water uptake and transpiration showed that the intensity of transpiration remained higher than the intensity of water uptake. This indicates that in the presence of MCPA in the root medium the relationship between the uptake and the loss components of water balance is not quantitatively equal, enboth in the light and in the dark. The existing disproportion results in the formation of a passive water balance of plants.     

Effect of Changes of Temperature Around Roots in Relation to Water Uptake by Roots and Leaf Transpiration

Effects of changes in temperature around roots on water uptake by roots and leaf transpiration were studied in Leucaena leucocephala (Lam.) de Wit., a subtropical woody plant species, and in Zea mays L. When the temperature around roots was rapidly lowered from 25 ℃ to 15 ℃, the water uptake by the roots and leaf transpiration were stimulated significantly within a short period ( 14 min). However, this effect did not occur when the cooling time was prolonged neither did if occur when the temperature around the roots was resumed from 15 ℃ to 25 ℃. Both the hydraulic conductivity of roots and leaf transpiration were increased substantially at first (within 20 min)and then decreased steadily to a level lower than those of the control in which the roots were continuous exposed to a low temperature ( 15 ℃ ). Low temperature also promoted the biosynthesis of ABA in roots and enhanced the xylem ABA concentration, but such stimulation did not occur untill about 30 min after cooling treatment, leaf transpiration was reduced markedly, but the hydraulic conductivity of roots increased when the root system was treated with exogenous ABA. It was suggested that some mechanisms other than ABA may be involved in the short-time cryostimulation of water uptake by roots and leaf transpiration.     

Water Relations,CO2 Exchange,Water-use Efficiency and Growth of Welwitschia mirabilis Hook. fil. in three Contrasting Habitats of the Namib Desert1

CO₂ exchange, transpiration and leaf water potential of Welwitschia mirabilis were measured in three contrasting habitats of the Namib desert. From these measurements stomatal conductance, internal CO₂concentration and WUE were calculated. In two of the three habitats photosynthetic CO₂ uptake decreased and transpiration increased with increasing leaf age while in the third habitat CO₂ uptake increased and transpiration decreased with leaf age. Except for the stomata of young leaf sections in this habitat, stomata closed with increasing δw leading to a pronounced midday depression of CO₂ uptake. The high stomatal limitation of photosynthetic CO₂ uptake of glasshouse-grown plants was verified in the natural habitat. Photosynthetic CO₂ uptake saturated between 800 and 1300 μmol photons m^?2 s^?1depending on leaf age and habitat. CO₂ uptake had a broad temperature optimum declining significantly beyond 32 °C. Predawn leaf water potential reflected water availability and atmospheric conditions in the three habitats and ranged from ? 2.5 to ? 6.2 MPa. There was a pronounced diurnal course of leaf water potential in all habitats. During the day a gradient in water potential developed along the leaf axis with the lowest potential at the leaf's tip. With respect to whole plant balances of CO₂ exchange and transpiration, there were marked differences between Welwitschias in the three habitats. Despite a negative CO₂ balance over a period of five months, leaves in the driest habitat grew constantly at the expense of carbon reserves in the plant. Only at the wettest site did carbon gain exceed carbon demand for growth. The WUE of whole plants was insignificant in all habitats. The results were as contrasting as the habitats and plants and did not allow generalisations about adaptational features of Welwitschia mirabilis.     

Water stress effects on leaf elongation,leaf water potential,transpiration, and nutrient uptake of rice,maize, and soybean

A pot experiment was conducted in the greenhouse to determine and compare the responses of rice (Oryza sativa L. var, IR 36), maize (Zea mays L. var. DMR-2), and soybean (Glycine max [L.] Merr. var. Clark 63) to soil water stress. Leaf elongation, dawn leaf water potential, transpiration rate, and nutrient uptake in stressed rice declined earlier than in maize and soybean. Maize and soybean, compared with rice, maintained high dawn leaf water potential for a longer period of water stress before leaf water potential. Nutrient uptake under water stress conditions was influenced more by the capacity of the roots to absorb nutrients than by transpiration. Transport of nutrients to the shoots may occur even at reduced transpiration rate It is concluded that the ability of maize and soybean to grow better than rice under water stress conditions may be due to their ability to maintain turgor as a result of the slow decline in leaf water potential brought about by low, transpiration rate and continued uptake of nutrient, especially K, which must have allowed osmotic adjustment to occur.     

Identification of rhythmic subsystems in the circadian cycle of crassulacean acid metabolism under thermoperiodic perturbations

Leaves of the Crassulacean acid metabolism (CAM) plant Kalancho? daigremontiana Hamet et Perrier de la Bathie show overt circadian rhythms in net CO2 uptake, leaf conductance to water and intercellular CO2 concentration, which are entrained by periodic temperature cycles. To probe their sensitivity to thermoperiodic perturbations, intact leaves were exposed to continuous light intensity and temperature cycles with a period of 16 h, applying a set of different baseline temperatures and thermodriver amplitudes. All three overt rhythms were analyzed with respect to their frequency spectra and their phase relations with the thermodriver. For most stimulation protocols, stomatal conductance and net CO2 change were fully or partially entrained by the temperature pulses, while the internal CO2 concentration remained dominated by oscillations in the circadian range. Prolonged time series recorded for up to 22 d in continuous light underline the robustness of these circadian oscillations. This suggests that the overt circadian rhythm of net CO2 uptake in CAM results from the interaction of two coupled original systems: (i) an endogenous cycle of CO2 fixation in the mesophyll, showing very robust periodic activity, and (ii) stomatal movements that respond to environmental stimuli independently of rhythmic processes in the mesophyll, and thus modulate the gas exchange amplitude.     

Effect on Transpiration and Water Uptake by Rapid Changes in the Osmotic Potential of the Nutrient Solution

The sudden changes in the rates of transpiration and water uptake which occurred when the osmotic potential of the nutrient solution surrounding the roots of young wheat plants was rapidly changed were studied. The transpiration was measured by the aid of the microwave hygrometer and the water uptake by a recording poto-meter specially built for this investigation. When the osmotic potential of the nutrient solution was rapidly increased by adding mannitol, there was a temporary transpiration increase. The maximum increase was greater but the total time of the temporary increase shorter when a higher mannitol concentration was used. The quantity of water transpired by the shoots due to the temporary transpiration increase seemed to be fairly constant irrespectively of the mannitol concentration. The water transport to the shoots was immediately reduced when the osmotic potential was rapidly increased. The immediate reduction was greater when a higher mannitol concentration was used. After the immediate reduction the rate of water transport increased without delay. When the osmotic potential of the nutrient solution was rapidly decreased by withdrawing mannitol there was a temporary transpiration decrease, and the water transport to the shoots was immediately increased. After this increase the rate of water transport started to decrease at once. When, however, the mannitol concentration had been 0.30 M or higher, the transpiration rate increased progressively, and the change of the rate of water transport was small. The results indicate that the primary effect of the rapidly changed osmotic potential is localized to the root surface. The rapidly reduced water transport to the shoots after adding mannitol brings about the temporary transpiration increase. The course of events after withdrawing mannitol is just the reverse to that when adding mannitol.